The invention relates to a procedure and agent for the colorimetric determination of nitrate ions in aqueous solutions.
The quantitative determination of nitrate ions is becoming of increasing importance in analysis. Nitrate in drinking water and service water, in foodstuffs, in agricultural chemistry and in general environmental analysis is increasingly becoming a general criterion of quality. Reliable and economic determination procedures are required for monitoring the limit values of nitrate in drinking water, for monitoring the fertilizer residues in foodstuffs and for determining the level of nitrate in soils which are used for agriculture in order to accomplish cost-effective mineral fertilizing.
Nitrate determinations are of great importance especially where there is suspicion of excessively high nitrate concentrations which are hazardous to health, in order to be able to prevent, for example, potentially carcinogenic nitrosamine formation.
Both physical and chemical methods for the determination of nitrate are known. The chemical methods are based either on the reduction of the nitrate to nitrite and the detection of the nitrite via the formation of an azo dye or on a nitration of phenolic compounds with the formation of colored aromatic nitro compounds.
On the one hand, carcinogenic reagents (for example hydrazine) or toxic heavy metals, which enter the environment with the sample wastes, are used in the reduction methods, and on the other hand, the methods based on the nitration of phenol derivatives in concentrated sulfuric acid are elaborate and are more or less subject to interference from chloride ions present in the sample.
Analyst 110, 313 (1985) discloses a spectrophotometric method for determining nitrate in mineral waters using resorcinol and concentrated sulfuric acid at 360 nm. The investigations showed that chloride ions have no effect on the analytical result up to a concentration of 500 mg/l; the deviations in the analytical result are +2.5, +6.3 and +9% at 1000, 2000 and 3000 mg/l respectively.
It is stated in Gewasserschutz, Wasser, Abwasser (GWA) 79, 74 (1985) that most of the tested organic amines and phenols and their derivatives proved to be unsuitable for a nitrate assay. In the case of catechol, mixtures of sulfuric acid and phosphoric acid are described as completely unsuitable. There is no interference with the determination using catechol in sulfuric acid by up to 1000 mg/l chloride ions.